Custom ESP-IDF Bootloader (Rollback Enabled)

This minimal project builds a custom ESP-IDF bootloader with rollback support enabled.
You can flash it later alongside a Rust firmware using `espflash`.

What this provides
- A minimal ESP-IDF project (CMake) that can build just the bootloader.
- Rollback support enabled via sdkconfig.defaults (CONFIG_BOOTLOADER_APP_ROLLBACK_ENABLE=y).
- A sample OTA partition table (partitions.csv) suitable for OTA and rollback (otadata + two OTA slots).
- A convenience script to build the bootloader for the desired target.

Requirements
- ESP-IDF installed and set up (IDF_PATH exported, Python env activated).
- A selected target (esp32, esp32s3, esp32c3, etc.).

Build
1) Ensure ESP-IDF is set up:
   source "$IDF_PATH/export.sh"

2) Pick a target (examples):
   idf.py set-target esp32
   # or use the script:
   ./build_bootloader.sh esp32

3) Build only the bootloader:
   idf.py bootloader
   # or using the script (which also supports setting target):
   ./build_bootloader.sh esp32

Artifacts
- build/bootloader/bootloader.bin

Using with espflash (Rust)
- For a no_std Rust firmware, you can pass this custom bootloader to espflash:
  espflash flash --bootloader build/bootloader/bootloader.bin \
                 --partition-table partitions.csv \
                 <your-app-binary-or-elf>

Notes
- Rollback logic requires an OTA layout (otadata + at least two OTA app partitions). The provided partitions.csv is a starting point; adjust sizes/offsets to match your needs.
- This project doesn’t build an application; it exists solely to produce a bootloader with the right configuration.
- If you need different log verbosity or features, run `idf.py menuconfig` and then diff/port the changes back into sdkconfig.defaults.
- Targets supported depend on your ESP-IDF version. Use `idf.py set-target <chip>` or `./build_bootloader.sh <chip>`.